In turbofan engines, fan blade fixing commonly takes the form of a dovetail joint. In conventional configurations, the base of the joint lies in a plane aligned parallel to the centerline of the engine. Because of highly sloped gas path characteristics, engines with axially aligned blade fixing require a platform on each fan blade to maintain the gas path profile established by the nose cone. The result is a significant distance between the hub outer diameter and the aft blade platform. This distance, combined with both the mass of the blade root spanning that distance and the mass of the blade platform, causes an additional load on the hub due to centrifugal forces which act on the blade as the hub assembly rotates. To compensate for the additional load, it is necessary to make the blade root "stockier" than the blade, which compounds the problem. These additional loads result in undesired stress within the hub.
A fan blade fixing configuration at an inclined angle relative to the engine centerline reduces the distance from the aft blade platform to the outer hub diameter. As a result, blade mass is reduced as well as the centrifugal force associated with the blade mass. There is a tradeoff, however. In conventional configurations with the blade fixing axially aligned, the centrifugal loading on the fan blade is almost entirely handled by the dovetail joint. This is possible because the centrifugal load appears solely as a radial force acting up through the fan blade. With the blade fixing at an incline, however, the centrifugal force vector resolves into both a normal force and a parallel force and it becomes necessary to secure the blade axially as well as radially in the hub.
Existing designs include securing the blade axially by a "hook" formed on the forward edge of the blade root. The hook contacts the hub, or an internal locking ring, thereby preventing the blade from traveling up the blade fixing incline. A serious disadvantage of this design is the tensile and bending stresses inherent in the hook configuration. To withstand these stresses, the mass of the hook must be substantial, especially for larger engines. Gains in the reduction of blade mass with the inclined configuration are lost as the hook is necessarily designed larger and larger with increasing engine size. Moreover, at some juncture hooks are no longer feasible from a physical constraint standpoint.